Bauxite and calcinated kaolin are the main minerals used for the production of aluminum oxide (alumina) and aluminum salts. Various methods are known in the art for carrying out this procedure, and some of them are described in Encyclopedia Kirk-Othmer (Volume 2; pages 252–271; 337) and Ullman (Volume A1, page 527–535).
The Bayer process is a traditional technology known for extracting aluminum hydroxide from bauxite by using caustic soda under pressure. This process comprises more than 15 stages and is a costly process. One problem with the aluminum sulfate produced from bauxite and kaolin, which is used mostly for the purification of water, is that it contains 0.3% to 0.8% Fe2O3 (Chemical Economics Handbook 1998). Hence, for the production of iron free aluminum sulfate of less than 100 ppm Fe2O3, which is used for example in the paper industry, pure aluminum hydroxide must be used. Many methods are known for extracting aluminum hydroxide from bauxite and from clay. Those from clay require a calcination step of considerably high temperature, usually higher than 1,100° C.
The Bayer process using bauxite for the production of aluminum hydroxide is known for 120 years. This process however, is very complicated and costly. Recently published, namely U.S. Pat. No. 5,993,758 to Nehari dated 1999, describes a process for the recovery of alumina and silica from aluminosilicate. This process comprises the steps of (a) heating the aluminosilicate to approximately 1000° C. with hydrated CaCl2 to obtain calcium-alumino-silicate and calcium-aluminate products, wherein the CaCl2 is substantially free of MgCl2; (b) leaching the products with HCl to form a solution comprising AlCl3 and CaCl2, and insoluble silica; (c) separating the insoluble silica from the solution; and (d) crystallizing AlCl3 from the solution and recovering the alumina from the crystallized AlCl3.
The process may further comprise the steps of: (e) substantially removing MgCl2 from the CaCl2 solution of step (d); and (f) recycling the CaCl2 solution for use in step (a). It is thus clear that the drawback of aforementioned process is costly. Thus, a more cost-effective process would therefore be beneficial for the recovery of alumina and silica from materials containing alumino-silicates. This process should avoid both calcination steps at high temperature, and further, should be insensitive to iron contamination in the process.